1. Field of the Invention
The presently disclosed subject matter relates to an image processing device, method, and an inkjet recording device, and in particular relates to a halftone processing technology utilized in ink ejection control of the inkjet recording device.
2. Description of the Related Art
Conventionally, a serial type inkjet recording device that controls ejection of each nozzle of a recording head on the basis of dot data obtained by halftone processing is known (see Japanese Patent Application Laid-Open No. 2002-096455 and Japanese Patent Application Laid-Open No. 2010-162770). The serial type inkjet recording device records images on a recording medium by repeating a scan operation of ejecting ink from a nozzle of a recording head while moving the recording head along a main scanning direction and a medium conveying operation which is sub scanning feed of intermittently conveying the recording medium in a sub scanning direction.
In such a recording method, behavior of impacted droplets on a recording medium, that is, dot behavior, is changed by recording position errors of individual dots to the recording medium by individual nozzles of a recording head, ejection amount errors of the individual nozzles, or a recording order or recording timing of dots or the like. Due to the change of the dot behavior on the recording medium, density irregularity called “banding” in which a recording density changes in a repetition cycle of individual print paths or a boundary of the individual print paths is conspicuous or the like is generated. When banding is generated, print image quality declines.
For such a problem, Japanese Patent Application Laid-Open No. 2002-096455 discloses an inkjet recording device that controls ejection of individual nozzles by thinning image data after halftone processing using a mask pattern corresponding to the individual nozzles when ejecting ink from the individual nozzles of a recording head. The inkjet recording device described in Japanese Patent Application Laid-Open No. 2002-096455 suppresses banding by setting the mask pattern such that a ratio of recording pixels (that is, a nozzle ejection rate) is lower for the nozzle near an end than the nozzle near a center part of a nozzle array of the recording head.
Also, Japanese Patent Application Laid-Open No. 2010-162770 discloses an inkjet recording device that controls ejection of individual nozzles of a recording head by performing halftone processing by ordered dither using a dither mask. In the inkjet recording device described in Japanese Patent Application Laid-Open No. 2010-162770, banding is suppressed by setting a threshold of the dither mask such that a nozzle using rate of at least one of nozzle groups (including most distal end nozzles arranged at both individual ends) arranged at each of both ends of a nozzle array of the recording head becomes lower than a nozzle using rate of an intermediate nozzle group arranged between the nozzle groups at both ends.
In a method of generating the dither mask used in Japanese Patent Application Laid-Open No. 2010-162770, first, a dot suppression threshold for suppressing recording of dots is set to specific pixels of the dither mask. Then, positions of the specific pixels to which the dot suppression threshold is applied are excluded, that is, a mask that excludes the specific pixels is applied, and a threshold of the dither mask is optimally arranged regarding remaining pixels (paragraph 0045 in Japanese Patent Application Laid-Open No. 2010-162770). Thereafter, by optimally arranging the dot suppression threshold to the excluded specific pixels, a target dither mask is obtained.
In Japanese Patent Application Laid-Open No. 2010-162770, as a term corresponding to “recording head” in the present specification, the term “print head” is used. Also, it is understood that “inkjet printer” in Japanese Patent Application Laid-Open No. 2010-162770 is a term corresponding to “inkjet recording device” in the present specification. It is understood that “nozzle using rate” in Japanese Patent Application Laid-Open No. 2010-162770 is a term corresponding to “nozzle ejection rate” in the present specification.